I wish I can source other threads, but someone replaced their Lumia 1520 with an iPhone 6 and then sold their iPhone for a decent amount of cash leftover after buying themselves a used 1520. This might be a route you can take in case a flagship comes by that you deem worthy. (: Besides, iPhone have such a great resell value.

There are a couple of factors you need to consider:
First is OEM: Understand that anything outside the Lumia series isn't dealing directly with MSFT which mean updates, availability, and upgrades.
Price: The Lumia 830 is cheaper especially with a promotion going on.
Upgrade: Remember the word on the street is Windows 10 will be coming to all Lumia devices running 8.1. While this does not mean that the M8 won't get Windows 10, it could mean a delay.
Personally I've owned an HTC Trophy (Windows phone 7) and a Lumia 920 and 1520. The Lumia brand just seems like it embraces the Windows ecosystem more.

I went with the HTC One M8 (free on Verizon with contract) after recently losing my Lumia 820. It's an outstanding phone but you do loose the Lumia apps and the glance screen. Here drive+ and maps work on it though. I considered the Lumia 830 as well at $69.99 on AT&T contract but the HTC seemed like a much more capable phone.